Examines the acute behavioral, EEG and hormonal changes that occur when humans are exposed to the physiological doses of the hormone at noon, early evening and late evening. This will test the relationship between efficacy of melatonin doses and circadian time. Levels of alertness and psychomotor performance will be continuously assessed. EEG patterns during 2 hour sleep test will be registered and blood samples will be taken via indwelling venous catheter for determination of serum melatonin levels.